In order to measure, for example, a temperature of a food load in a baking oven and to relay it to a household appliance controller, DE 10 2005 015 028 describes the use of high-frequency electromagnetic waves to excite surface wave devices incorporated in a temperature measuring probe so as to infer the food temperature indirectly from the electromagnetic waves returned from the surface wave devices. In the process, high-frequency electrical signals are generated in the household appliance controller and transmitted as electromagnetic waves via a transmitting antenna toward the temperature measuring probe. The waves returned from the temperature measuring probe are received by the transmitting antenna, which is at the same time a receiving antenna, and passed to an evaluation circuit. In order to infer the food temperature from the received electrical signals, the system sweeps through a predetermined frequency band each time a temperature measurement is made, i.e., each time a measurement cycle is performed. Electrical signals of different frequencies within the band of frequencies are successively generated in the household appliance controller, transmitted as electromagnetic waves to the temperature measuring probe, and the electromagnetic waves returned therefrom are in turn converted to electrical signals. The food temperature is then determined indirectly from the highest signal level of the then available electrical signals and the transmit frequency associated therewith, which is the resonant frequency.
The quality of the electromagnetic waves returned from the temperature measuring probe and of the electrical signals that are generated therefrom in the evaluation circuit depends strongly on the coupling characteristics between the temperature measuring probe and the household appliance controller. Such coupling characteristics include, for example, constantly changing ambient conditions, in particular in a treatment chamber in the form of a cooking chamber, and the orientation of the temperature measuring probe, i.e., of its antenna, with respect to the receiving antenna of the household appliance controller. During the measurement cycle, an error may occur depending on these or other possible coupling characteristics. For example, the levels of the electrical signals received by the household appliance controller may be too high or too low. In such cases, the food temperature can no longer be reliably determined with the desired accuracy.
A method for automatically adapting a signal level between an input and an output of an electrical circuit is described in DE 102 48 698 A1. However, this method relates only to individual electrical signals, and not to a measurement cycle.